from __future__ import annotations

from pathlib import Path
import struct


SECTOR_BYTES = 512
PARTITION_LBA = 1
PARTITION_SECTORS = 8192
RESERVED_SECTORS = 1
FAT_COUNT = 2
FAT_SECTORS = 32
ROOT_ENTRIES = 512
ROOT_SECTORS = ROOT_ENTRIES * 32 // SECTOR_BYTES
DATA_RELATIVE_SECTOR = RESERVED_SECTORS + FAT_COUNT * FAT_SECTORS + ROOT_SECTORS


def build_fixture(path: Path, config_file: bytes) -> Path:
    if len(config_file) != 64 * 1024:
        raise ValueError("配置文件必须为 64 KiB")
    image = bytearray((PARTITION_LBA + PARTITION_SECTORS) * SECTOR_BYTES)
    image[510:512] = b"\x55\xaa"
    partition_entry = 446
    image[partition_entry + 4] = 0x06
    struct.pack_into("<II", image, partition_entry + 8, PARTITION_LBA, PARTITION_SECTORS)

    boot = PARTITION_LBA * SECTOR_BYTES
    image[boot : boot + 3] = b"\xeb\x3c\x90"
    image[boot + 3 : boot + 11] = b"MSDOS5.0"
    struct.pack_into("<H", image, boot + 11, SECTOR_BYTES)
    image[boot + 13] = 1
    struct.pack_into("<H", image, boot + 14, RESERVED_SECTORS)
    image[boot + 16] = FAT_COUNT
    struct.pack_into("<H", image, boot + 17, ROOT_ENTRIES)
    struct.pack_into("<H", image, boot + 19, PARTITION_SECTORS)
    image[boot + 21] = 0xF8
    struct.pack_into("<H", image, boot + 22, FAT_SECTORS)
    image[boot + 510 : boot + 512] = b"\x55\xaa"

    fat = bytearray(FAT_SECTORS * SECTOR_BYTES)
    struct.pack_into("<HH", fat, 0, 0xFFF8, 0xFFFF)
    clusters = len(config_file) // SECTOR_BYTES
    for index in range(clusters):
        cluster = 2 + index
        struct.pack_into("<H", fat, cluster * 2, 0xFFFF if index == clusters - 1 else cluster + 1)
    first_fat = boot + RESERVED_SECTORS * SECTOR_BYTES
    image[first_fat : first_fat + len(fat)] = fat
    image[first_fat + len(fat) : first_fat + 2 * len(fat)] = fat

    root = boot + (RESERVED_SECTORS + FAT_COUNT * FAT_SECTORS) * SECTOR_BYTES
    image[root : root + 11] = b"MSCCFG  BIN"
    image[root + 11] = 0x20
    struct.pack_into("<H", image, root + 26, 2)
    struct.pack_into("<I", image, root + 28, len(config_file))
    data = boot + DATA_RELATIVE_SECTOR * SECTOR_BYTES
    image[data : data + len(config_file)] = config_file
    path.write_bytes(image)
    return path


def first_fat_offset() -> int:
    return (PARTITION_LBA + RESERVED_SECTORS) * SECTOR_BYTES